Thanks for mentioning.
Indeed, as you describe, firefox v51 plays the first song and subsequent songs don’t play (output sound), the seek bar is moving, but no sound…
It has something to do with the sound visualizations (Web Audio Api).
If you turn off the visualizations, then it all works again.
To turn off, go to “settings” and uncheck “Enable music visualisations.”
(Or use another browser off course…)
This is just a quick way to be able to play songs.
I’ll have to investigate it further, and come back when I found something.